(a) Field of the Invention
The present invention relates to a logic circuit for preventing erroneous operations of photoelectric switches, and it is intended to obviate erroneous operations caused by noises, while minimizing the packaging area of the whole circuit. Among the inventive photoelectric switches, which include both of the transmission type and reflection type, the transmission-type photoelectric switch is designed to synchronize the timing signal of the light reception circuit with the pulse signal of the light projector without using wires, and the reflection-type photoelectric switch is designed to prevent mutual interferences with confronting counterparts.
(b) Description of the Prior Art
Most of photoelectric switches operate by detecting a signal of a light emitting diode which is driven by a pulse generation circuit to emit intermittent lights. Contact-type switches create spark noises intermittently at opening and closing their points, and the light receiver of photoelectric switch fails to discriminate these noises from the legitimate signal, resulting often in erroneous operations. In order to prevent such malfunctions, the synchronous detection system is generally adopted. However, this system can merely lower the probability of erroneous operations caused by noises, and it cannot completely prevent a malfunction when a noise coincides with the signal. To cope with this matter, the voltage level as a result of synchronous detection is integrated by an integration circuit which is mainly composed of a capacitor, and the detected signal is validated only when the integrated voltage has reached a certain voltage level. Namely, according to the error preventive system employed widely, the signal detection is not recognized unless signals are received a number of times continuously.
Photoelectric switches are provided in their circuit with a short-circuit protection circuit with the intention of protecting the output transistor from destruction which would occur if the signal line is erroneously short-circuitted to the power line. However, if the output line is extended too long, it picks up noises to activate the short-circuit protection circuit, which erroneously invalidates the output. Use of a photoelectric switch in an noise-rich environment will encounter frequent malfunctions of the short-circuit protection circuit. On this account, in such an environment, the short-circuit protection circuit is provided with a capacitor-based integrating circuit so that the output circuit has a slower response, thereby preventing erroneous operation.
Recently, photoelectric switches have their detection circuit formed mostly of an integrated circuit (IC) so that they are compact, and moreover attempts are being made to incorporate the whole photoelectric switch in a single IC chip inclusive of all necessary circuits. However, if the above-mentioned two integrating circuits for noise prevention use capacitors, each being at least several hundred pF in need, the integrated circuit will require a very large packaging area for these capacitors, which ruins the meaning of the single chip configuration. An alternative configuration of externally connected capacitors causes the IC chip to have increased terminals and thus a greater external dimensions, conflicting with the purpose of circuit integration in pursuit of compactness. In addition, the integrating circuit has a long fall time, creating a time lag of the detection output from the pulse reception, and therefore it does not ensure the high accuracy operation.
Transmittion-type photoelectric switches are installed at separate locations for their light projector and light receiver, and in order to achieve enhanced detection accuracy, the synchronous detection system in which the light projector and receiver operate at a coincident timing must be employed. On this account, the conventional design technique uses a gate circuit in the light receiver and, in addition, lowers the response speed so that a moderate signal-to-noise ratio is retained, at the sacrifice of accuracy due to the lowered response speed. Therefore, innovative means of enhancing the accuracy without dulling the response has been desired.
Use of two reflection-type photoelectric switches confronting each other imposes such an interference problem that one photoelectric switch can malfunction due to the presence of the light projector of the confronting photoelectric switch. Conventionally this problem is obviated by using light projector of different light emission periods in case of a same model, under obligation of readjustment and intricate inventory control for the products.